Pierre qui roule...

une pierre = a rock (pee - ay - re)

jeter la pierre à quelqu'un = to accuse someone (literally to throw a rock at someone - jeter means to throw)
jeter la première pierre = to cast the first stone
apporter sa pierre à quelquechose = to make a contribution to someting (literally to bring one's rock to something)
faire d'une pierre deux coups = to kill two birds with one stone
un jour à marquer d'une pierre blanche = a Red letter day (literally a day to be marked with a white stone)